The present invention relates in general to microelectronic elements and methods of making same, and more particularly, to interconnection structures having bondable leads made using positive photoresist material.
Microelectronic elements such as semiconductor chips are connected to external circuitry, such as the circuitry of a supporting substrate or circuit panel, through electrical contacts on the front face of the chip. Various processes for making these interconnections use prefabricated arrays of leads or discrete wires. For example, in tape automated bonding processes, a dielectric supporting tape such as a thin film of polyimide, includes an array of metallic leads on one surface of the dielectric film. The metallic leads are aligned with the contacts on the front face of the chip. The dielectric film is juxtaposed with the chip so that the leads extend over the front or contact bearing surface on the chip. The leads are then bonded to the contacts of the chip, such as by ultrasonic or thermocompression bonding. The terminals on the dielectric film may then be connected to external circuitry for electrically interconnecting the chip and the external circuitry.
In various microelectronic packages, it is often desirable to provide a connection between two components, which can accommodate relative movement between the components. For example, where a semiconductor chip is mounted to a circuit board, thermal expansion and contraction of the chip and circuit board can cause the contacts on the chip to move relative to the corresponding electrically conductive features of the circuit board. This can occur during service and can also occur during manufacturing operations as, for example, during soldering operations on the circuit board.
As illustrated in U.S. Pat. No. 5,518,964 (“the '964 patent”), the disclosure of which is incorporated herein by reference, movable interconnections between elements such as a semiconductor chip and another element can be provided by first connecting leads between the elements and then moving the elements away from one another through a preselected displacement so as to bend the leads. For example, a connection component may incorporate a dielectric body and leads extending along a bottom surface of the dielectric body. The leads may have first or fixed ends permanently attached to the dielectric element and connected to electrically conductive features such as terminals, traces or the like on the dielectric body. The leads may also have second ends releasably attached to the dielectric body. The dielectric body, with the leads thereon, may be juxtaposed with the chip and the second ends of the leads may be bonded to contacts on the chip.
Following bonding, the dielectric body and chip are moved away from one another, thereby bending the leads towards a vertically extensive disposition. During or after movement, a curable material such as a liquid composition is introduced between the elements. This is cured to form a compliant dielectric layer such as an elastomer or gel surrounding the leads. The resulting packaged semiconductor chip has terminals on the dielectric body connection component which are electrically connected to the contacts on the chip but which can move relative to the chip to compensate for thermal effects. The packaged chip may be mounted to a circuit board by solder-bonding the terminals to conductive features on the circuit board. Relative movement between the circuit board and the chip due to thermal effects is taken up in the moveable interconnection provided by the leads and the compliant layer.
In order to achieve bonding of the leads to the contacts on the chip, a bondable material such as tin or tin alloys is deposited onto the second ends or tips of the leads. The bondable material can be deposited by electroplating processes using a patterned masked material and plating solder through the formed openings in the mask. However, electroplating processes are not compatible in certain microelectronic components where the leads are not all electrically interconnected to a common bus. This is often the case in the aforementioned microelectronic components.
It is also known to deposit bondable material onto the tips of leads using conventional solder and reflow application techniques. The tips of the leads are provided with a metal pad which acts as a diffusion barrier so that the lead material, typically copper, does not diffuse into the bondable material, as well as to promote wetting of the bondable material during the reflow process. In forming the leads and pads, there is required the multiple application of a photoresist mask which is patterned to allow for the sequential deposition of the metal layers forming the leads and pads. Due to the projection of the pads, there is a limitation as to the application equipment which can be used for depositing the photoresist material. For example, the projecting pads will interfere with certain roller coating equipment known for applying photoresist material.
Accordingly, there is the need for improvements in methods of making microelectronic components having bondable leads by application of solder material to the lead tips. There is further the need for improvements in methods of making microelectronic components which provide greater flexibility in photoresist application. Still further, there is the need for methods of making microelectronic components which use a single photoresist mask for forming leads and solder ball pads.